 In the classical structural rigidity theory, one considers a collection of rigid bars connected by joints allowing free rotations around the joint axis (bar-and-joint frameworks). One is then interested in understanding what are the allowable motions of the framework, i.e., those motions that preserve the lenghts of the bars and their connections to the joints. The so-called trivial motions for these frameworks are precisely the rigid body translations and rotations of the system. For some frameworks, there may be additional motions, known as flexes, that also preserve the constraints. This is captured by the notion of infinitesimal motions of the framework and is characterized by the rigidity matrix of the framework [1]}.

